I think I'd prefer a flat timespan rather than a lifetime-dependent one. The two flaws I see with the lifetime-dependent one are:

• It can give wildly different opportunity to the rightsholder depending on how old they are and what their random life circumstances happen to be. A 20-year-old author could have 80 years' hold on their work whereas a 70-year-old one could have just 10. Unless Truck-kun randomly gets involved and sends that 20-year-old author into another world a day after he published.

• It creates an incentive to assassinate popular authors.

It also creates complexity for work-for-hire situations where a corporation owns a copyright, though that's already a special case so one could continue handling it separately.